To join or make contact with a language team, locate your Language Team's forum section and introduce yourself there.
If a Language Team you wish to join does not yet exist, post your desire to start (and perhaps coordinate) one:

Text-based focus:
Our main tool for translating text materials (Pootle):Current list of Pootle projects
Please do not select English Proofreading or French Chapter unless you are a member of that language team. Also, do not select Terminology. These three are non-translatable.

Video-based focus:
Our main tool for translating video materials (dotSUB)
A list of videos currently being processed by the teams (we call them "working locations" and the links should NEVER be passed out to the public):

A list of locations where we can find finished translations of the videos the teams worked on (we call it the repository and these are very much meant for public use & distribution)